This invention relates to a shadow mask of a color cathode ray tube (CRT), and also a method of manufacturing the shadow mask.
Recently it has been increasingly demanded that a television screen provide comfortable viewing. ("Comfortable viewing" is a term of art referring in part to the ability to discern fine characters and images on the screen, i.e., high resolution, and in part to a brighter picture produced by increasing beam current.) Therefore, it is now necessary to enhance the resolution of a CRT. In order to provide a CRT of high resolution, the shadow mask of the CRT must have smaller beam-guiding holes in greater numbers, which are arranged in a higher density.
In a color CRT, a shadow mask is provided near the tricolor fluorescent screen located inside the front end element of the CRT. The electron beams emitted from the electron guns pass through the beam-guiding holes cut in the shadow mask, and are applied to the tricolor fluorescent screen. To apply the electron beams to accurate positions on the fluorescent screen, thereby to form high-quality images, it is necessary to cut beam-guiding holes in a metal plate with a high precision, so that the holes take accurate positions and have desired diameter and shape. Further, in order to balance the size of the image formed on the center of the screen with the size of the image formed on the peripheral portion of the screen, the shadow mask must be pressed to have its peripheral edges curved with a predetermined curvature. If the beam-guiding holes are cut with an insufficient precision, or if the shadow mask is pressed into an unaccurate curved form, the beam-guiding holes will not be aligned with the positions at which the electron beams should fall on the fluorescent screen. Consequently, a phenomenon called "doming" will occur, inevitably deteriorating the quality of the image formed on the screen.
Generally, a shadow mask is manufactured in the following way. First, an ingot is hot-rolled, then cold-rolled into a thin, band-link sheet. Thereafter, a number of elongated holes are perforated in the sheet by means of etching. A rectangular shadow mask plate having a desired area is cut out of the band-like sheet.
Hitherto, the cutting plan of the shadow mask plate is laid out such that the short sides of the rectangular plate extend in the cold-rolling direction of the band-like sheet. The elongated holes are so arranged that they are parallel to the short sides of the shadow mask plate. Hence, the longitudinal axes of the elongated holes are parallel to the rolling direction of the band-like sheet. After the elongated holes have been cut by etching, the shadow mask plate is pressed into a desired shape.
Most shadow masks are made of aluminum-killed steel. This is because this material is easy to etch and shape. Another reason is that an oxide layer, which reduces the reflection of electron beams, can easily coated on a plate made of a aluminum-killed steel.
However, aluminum-killed steel has a relatively great coefficient of thermal expansion. When a shadow mask made of aluminum-killed steel and having tiny and densely arranged holes is heated due to the application of electron beams, it expands, thus deforming the holes, and ultimately giving rise to a local doming.
In view of this, it has recently been proposed that a shadow mask having tiny and densely arranged holes be made of Invar alloy having a small coefficient of thermal expansion, in particular, so-called Invar (or Nilvar) alloy, i.e., a 64Fe36Ni alloy. ("Invar" is a trademark with registration Number 63970.)
It is disclosed in U.S. Pat. No. 4,528,246 and U.S. patent application Ser. No. 647,924 now U.S. Pat. No. 4,665,338 that shadow masks are made of Invar alloy. In both cases, use is made of the small coefficient of thermal expansion of this specific alloy, for the purpose of minimizing the expansion of the beam-guiding holes during the use of the shadow mask.
Japanese Patent Disclosure No. 59-101743 also discloses that a shadow mask is made of Invar alloy. This publication further teaches that an Invar alloy sheet expands at the lowest rate in the direction at 45.degree. to the rolling-direction of the sheet. For this reason, the beam-guiding elongated holes of the shadow mask disclosed in this publication are arranged such that their axes extend in the direction at 45.degree. to the rolling direction of the sheet.
Invar alloy has a great 0.2% proof stress (The term "0.2% proof stress" means the nominal stress applied on material, leaving a 0.2% plastic strain in the material). Therefore, when a plate of Invar alloy is pressed, its springback is great. Even if a shadow mask plate is pressed in such a shape as is shown in FIG. 5 under stress, its edge portions will return to their original shape upon release of stress, as is shown by two-dot, one-dash line in FIG. 5. It is difficult to press mask plate 5a made of Invar alloy into the desired form shown by solid line in FIG. 5. Thus, it is necessary to anneal the shadow mask plate in vacuum or in a hydrogen atmosphere, thus reducing the 0.2% proof stress of the alloy, before the plate is pressed. Once the plate has been annealed, its springback is minimized, whereby the plate can be easily pressed into the desired form.
However, when the shadow mask plate of Invar alloy is pressed into the curved form (as solid line shown in FIG. 5) after the 0.2% proof stress of the alloy has been minimized, elongated holes 6 of the plate will likely explained along their lateral axes. Hence, as is shown in FIG. 6, the width of each elongated hole 6 increases. Consequently, the dimensional precision of holes 6 is low.